I don't get why people see it as a happy ending?
Walt and Hank are dead. Yes Walt got revenge but at the ultimate cost, it was a pyrrhic victory if it was anything.
Marie lost her husband and is estranged from her sister, she's basically alone as we never see her with other friends or family really.
Skyler lost her husband, her house, her livelihood and has trouble with the law.
Flynn will get the money but lost the person he respected the most and has to live in the shadow of his actions, Plus Vato hates him.
Holly will grow up with the stigma of who her father is.
Saul had to go into hiding.
Jesse may have had a 'win' by escaping the Nazi's and killing Todd but the emotional scars will haunt him forever, he's pretty much broken.
And that's what I think the show was about really, not whether all the Nazi's were killed but at what cost which means Walt had to win because otherwise the guilt of all that happened on the show would be shared with the Nazi's, this way we see Walt as the last man standing (kind of) so all the responsibility lands on his shoulders.
Or at least that's my initial reaction, just finished watching it and I think I may watch again later.